1. Field of the Invention
The invention relates generally to the field of power conversion, more specifically the invention relates to control of a power converter.
2. Description of the Related Art
Early high frequency switching power conversion devices operated without feedback or in an open-loop, “chopper”, switch-mode. Later, it was learned that output regulation could be improved by adding a sense point at the converter output. By using the signal at the sense point, the high frequency switch duty cycle can be increased or reduced depending upon whether the output voltage was lower or higher than a desired load voltage.
The common method of voltage feedback control consists of feeding a feedback signal into a gain-compensated operational amplifier that compares the signal to a precision reference. Any deviation of the signal from the reference is amplified by the amplifier (usually referred to as the error amplifier) at high gain and then output to a switching regulation control circuit. The switching regulation control unit adjusts the switching duty cycle by an amount controlled by an error signal resulting from the demand or lack of demand for power output. This is usually accomplished by mixing the error amplifier output signal with a ramp oscillation signal resulting in a pulse with a width controlled by the error signal. This pulse is then used to modulate the switch on-time and thereby control the duty cycle and the output voltage of the converter. This technique is called pulse width modulation (PWM).
Other techniques for voltage feedback control add additional signals to the output of the error amplifier before it produces the final drive pulse (but not before the input to the error amplifier.) These additional signals are, for example, signals representing a switch peak current. If the switch exceeds a certain current threshold, the drive pulse would be terminated. This method is called current mode control, and the converter still uses the external voltage feedback control loop. Other techniques add signals representing output inductor currents for current sharing in so-called multi-phase converters.
Regardless of the type of design of a switching converter, there is still a basic problem when a remote load voltage is sensed with a feedback loop, because the voltage at a remote load is usually less than the voltage regulated at the terminals of the converter due to a drop in transmission line voltage. Some attempts have been made to sense the remote load voltage, via a signal transmission line, to regulate the converter to provide the correct voltage at the load. However, this approach introduces a new problem in that the voltage signal fed back from the load is delayed, and possibly phase shifted, from the actual signal at the load due to the long transmission line used to feedback the voltage signal.
Furthermore, when remote sensing is not at the power converter's terminals, the terminals of the power converter can swing wildly in response to the remote load demands. If other loads were connected to the same converter, but not as far away as the remote sensing, these loads could suffer from over-voltage excursions or even continuous over-voltage conditions due to the sensing at the farthest remote load (this is called a multiple load problem.) And even worse, due to the delay and phase shift of the transmission lines in the complete circuit, the converter can become unstable due to inadequate overall feedback phase margin.
Most AC to DC converters have primary to secondary side isolation for safety reasons, and usually the voltage sense feedback loop contains an isolation device such as an optically isolated coupler in an integrated circuit to provide the required safety isolation. Unfortunately, this feature does not help to reduce the remote sensing problem. Still there is a need to sense voltage at remote loads without introducing instability.
Another refinement of the remote sensing technique is also introduced where both the remote voltage and the remote common potential signals are fed back as a type of differential feedback signal. This refinement has the advantage of rejecting common mode noise, but it does not address the phase shifting, delay, and multiple load control problem associated with remote sensing.